In a fluid coupling of this kind, the coupling slip can be adjusted during operation. Thus, with approximately constant rotary input speed, the rotary take-off drive speed of the coupling may also be adjusted.
In one known fluid coupling of this type (see German Offenlegungsschrift No. 26 12 133) the control device revolving with the secondary coupling half has a power comparing element and equalizer comprising a control piston which can be displaced in the radial direction, relative to the rotary axis of the coupling. This piston is loaded on one side by a centrifugal force (an operating value), which is dependent upon the output speed (a controlled variable). The piston is loaded on the other side, counter to the centrifugal force, by a variable spring force which can be adjusted by means of a temperature sensor and which provides an adjustable and variable command value. To adjust the rotary drive output speed when the operating or controlled variable value differs from the command value, the following provision is made:
The control piston simultaneously forms the movable valve element of a control valve which controls a flow of pressure medium extending to a regulating piston. The regulating piston displaces an annular or cylindrical slide valve, which projects into the working chamber of the coupling, thereby restricting to a greater or lesser extent the circulation of fluid in the working chamber.
In another embodiment of a known coupling the control device varies the degree to which the working chamber of the coupling is filled.
A known fluid coupling may be used in particular for driving a cooling fan. In this instance, it is possible to adjust the rotary drive output speed of the fluid coupling as a function of a change in temperature, a change which can be measured directly at the rotating parts of the coupling, in order thereby to adapt the output flow of the cooling fan automatically to the cooling requirement.
This known fluid coupling arrangement is not, however, (at least without involving other measures), capable of regulating the rotary take-off drive or output speed of the fluid coupling to a value which is preset by an adjustment device which is separate from the rotating parts of the couplings, i.e. one which depends on a command value which is to be transmitted to the coupling from an outside source. However, the latter type of operation is necessary for many applications of fluid couplings, including the driving of a conveyor belt, the operating speed of which should be capable of being set from a control center, or the driving of a pump, the supply pressure or supply flow of which is to be held at a given value by varying the rotary speed.
In all of these cases, it is difficult to transmit the command value from a remote location to the fluid coupling in such a way that a precise, e.g. linear, relationship is established between the command value and the rotary take-off speed of the coupling. The difficulty lies in the fact that the command value may be falsified during its transmission from a stationary part to a rotating part, or, after being transmitted to a rotating part, as a result of a change in the rotary speed, or may be changed after transmission to the rotating part possibly with variation in the speed of rotation.
A known method of controlling the filling level and thus the rotary take-off or output drive speed of a hydrodynamic coupling according to a command value supplied externally consists of providing an external working fluid circuit, i.e. continuously removing working fluid from the coupling and continuously supplying working fluid which is preferably cooled and, for example, varying the amount of working fluid conducted away. For this purpose, an outlet valve revolving with the coupling may be provided (see German Pat. No. 1,600,974). However, this requires a relatively complicated adjustment mechanism.
It is also known to provide an adjustable scooping tube (see the publication "Voith Getrieberegelkupplungen," Cr 101, especially Page 10). In this instance, the hydraulic scooping tube control is actuated from an adjustment gear via a cam plate. With the aid of the cam plate, it is possible for a specific setting of the adjustment gearing to be associated with a specific rotary drive speed. The disadvantage of this construction lies in the relatively high cost of the scooping tube and its control and in the fact that it is frequently very difficult to determine the correct shape for the cam plate.
Finally, hydrodynamic couplings with external circuits are known for example, see German Offenlegungsschrift No. 26 14 476, in which the flow of working fluid supplied can be varied by means of an inlet valve. However, even in this type of coupling, it is very difficult to establish a precise relationship between the command value (the flow of fluid supplied) and the rotary drive speed. It is impossible to achieve a precise proportional relationship.